Penicillin discovered by Alexander Fleming

Penicillin was discovered by chance, after Fleming accidentally left a dish of staphylococcus bacteria uncovered for a few days. He returned to find the dish dotted with bacterial growth, apart from one area where a patch of mould (Penicillin notatum) was growing. The mould produced a substance, named penicillin by Fleming, which inhibited bacterial growth and was later found to be effective against a wide range of harmful bacteria. However, it was not until World War II that penicillin, the first antibiotic, was finally isolated by Howard Florey and Ernst Chain. Fleming, Florey and Chain received a Nobel prize in 1945, for their discovery which revolutionised medicine and led to the development of lifesaving antibiotics.

Further info:

Time Life - Alexander Fleming -